This invention relates a process for fabricating and thermoforming prestressed plastic articles, and to articles made by the process.
In thermoforming, a sheet of thermoplastic resin is clamped into a clamp frame and heated. An oven or heat station is utilized, which heats the edges as well as the center of the thermoplastic sheet so that, when it is subsequently delivered to a forming station, an overall temperature balance has been attained. After heating, the sheet is conveyed to the forming station where by one of several methods, it is forced over a mold. In any thermoforming operation residual heat of the plastic must be removed as quickly as possible after forming. After cooling the end product is removed from the forming station and sent to a trim press where it :s trimmed from the web.
It can be seen that the thermoforming operation is utilized for making articles from sheets of thermoplastic resins, for example, acrylics (acrylonitrile polymers), polycarbonates, polyesters, polyimides, polypropylene, polystyrene and polyvinyl alcohol. One of the disadvantages of the process is that, using thermoplastic resins, it is not always possible to obtain the strength properties desired. As an example, thermoforming has been attempted for bodies of certain airborne objects. However it was found that such bodies readily cracked on landing. In accordance with the practice of this invention a thermoforming process has been developed which leads not only to plastic articles having better strength properties, but to prestressed plastic articles.
U.S. Pat. Nos. 4,194,938; 4,201,612; and 4,230,764 disclose methods for forming prestressed plastic articles by thermoforming, wherein a sheet of a thermoplastic resin is forced over a mold, the methods including heating a thermoplastic sheet and a thermosettinq resin-impregnated sheet of fiber-reinforced weft under conditions such that the thermoplastic layer would normally follow the mold, superimposing the soft thermoplastic sheet on the thermosetting resin layer, forcing the thermoplastic layer and thereby the thermosetting layer throughout the mold to put the thermosetting layer under tension, cooling the thermoplastic layer during the curing of the thermosetting layer to also put the thermoplastic layer under tension, and curing the thermosetting resin while both layers are under tension to form the prestressed plastic article.
While the methods of these patents have satisfactorily produced prestressed plastic articles, it has not been possible to consistently produce flat panels or shaped panels of a predetermined curvature with the use of these methods or article constructions. Accordingly, a need has arisen for a prestressed plastic article construction and method that can produce such articles in a flat configuration or in a predetermined curved or shaped configuration. The article and method of the present invention meet this need.